The present invention relates to an apparatus for monitoring a condition of a plurality of electrically energizable devices. The invention is applicable to the monitoring of conductive, capacitive and inductive electrical devices.
When an initial condition of an electrically energizable device or combination of devices changes to a faulty condition, an electrical power consumption of the electrically energizable device in the faulty condition usually differs from the electrical power consumption under corresponding conditions of the device in its initial condition. For example, in the case of a light-bulb, its power consumption will probably fall to zero if the filament of the bulb fails and a same operating voltage continues to be applied to the bulb. In the case of a number of light bulbs connected together in parallel with one another, the aggregate power consumption of the bulbs will fall by a relatively small amount if the filament of only one bulb fails. In the case of a transformer or other inductive device, its power consumption may increase if the condition of the device deteriorates and the same operating voltage continues to be applied to the device. In the case of an assembly comprising an electric motor and a device driven by that motor, failure or deterioration in the condition of the driven device may bring about either an increase or a decrease in the power consumption of the motor, depending upon the nature of the failure. Changes in conditions other than failures or deterioration of devices also affect the power consumption of electrically energizable devices. For example, a change in the temperature of a device may affect its power consumption.
It has been proposed that the condition of an electrically energizable device should be monitored by monitoring the current flow through the device. For example, there is disclosed in GR 2,150,372A a system for detecting failure of lamps in a traffic light. This specification proposes that a value representing the current flow through a group of lamps which are energized concurrently should be stored in a memory during one cycle of the traffic light's operation. A corresponding new value of current flow measured during the next cycle should be compared with the stored value. If these values agree within a predetermined tolerance, then the new value is substituted in the memory for the previously stored value. If there is no agreement within the predetermined tolerance, then an alarm is given. The published specification also explains that the lamps are dimmed for operation during hours of darkness by reducing the applied voltage by 30%. It is suggested that an indication of this voltage reduction should be compared with another memory value so that the corresponding variation in current through the lamps will be taken into account.
One of the problems which arises in monitoring electrically energizable devices is that the applied voltage can vary for reasons other than deliberate dimming of lamps and these variations are likely to be smaller than 30%. In a traffic light system where a group of devices is monitored by measuring the aggregate current flow through those devices, it is desirable for the monitoring system to be capable of distinguishing between a change in current flow which results from failure of one of the devices of the group and a change in current flow which results from a change in the applied voltage.